Visualising a viral RNA genome poised for release from its receptor complex.

We describe the cryo-electron microscopy structure of bacteriophage MS2 bound to its receptor, the bacterial F-pilus. The virus contacts the pilus at a capsid 5-fold vertex, thus locating the surface-accessible portion of the single copy of the pilin-binding maturation protein present in virions. This arrangement allows a 5-fold averaged map to be calculated, showing for the first time in any virus-receptor complex the nonuniform distribution of RNA within the capsid. Strikingly, at the vertex that contacts the pilus, a rod of density that may include contributions from both genome and maturation protein sits above a channel that goes through the capsid to the outside. This density is reminiscent of the DNA density observed in the exit channel of double-stranded DNA phages, suggesting that the RNA-maturation protein complex is poised to leave the capsid as the first step of the infection process.